Extensive preclinical studies have implicated the chemokine receptor, CCR2, in numerous inflammatory diseases including multiple sclerosis and other neurodegenerative diseases, neuropathic pain, rheumatoid and osteoarthritis and fibrosis. However, at this time, no drugs targeting CCR2 have made it through clinical trials largely due to efficacy issues. Several reasons for lack of efficacy have been suggested including redundancy of the chemokine system such that additional receptors may need to be targeted simultaneously for complex disease indications, and non-optimal occupancy of the target receptor in vivo because of fast compound off- rates. To deal with redundancy, dual targeting of both CCR2 and CCR5 is being pursued. To improve receptor occupancy of potential lead compounds, long residence time (LRT, e.g. low off-rate) compounds are also under development as their efficacy is predicted to exceed short residence time compounds with equivalent affinities for CCR2. Such LRT compounds may also lead to improved drug safety because of lower or less frequent dosing required to achieve efficacy. Finally, allosteric antagonists of CCR2 are also attracting increasing interest because of their potential to regulate effects of orthosteric modulators in a titratable way. Despite the great progress towards the development of these novel long-residence, dual-targeting, and allosteric modulators of CCR2, the structural basis of their action remains unknown. This is because as a membrane protein from the GPCR family, CCR2 is a highly challenging crystallization target. The objective of the present proposal is therefore to determine crystal structures of CCR2 in complex with (i) long residence time orthosteric antagonists, (ii) allosteric modulator(s) and (iii) dual CCR2/CCR5 orthosteric antagonists. This work builds on our recent success in determining the structure of CXCR4 in complex with a chemokine, which is the first structure of any GPCR with a protein ligand, and on strong preliminary data in the form of stable homogenous complexes of CCR2 with all three types of modulators. Our central hypothesis is that compound kinetics, selectivity, and allostery originate from specialized atomic level interactions with the receptor, and that structure determination will reveal these interactions and thus enable rational design of compounds with optimized properties. Our long term goal is to aid in the development of drugs that target CCR2 and other therapeutically valuable receptors. The significance of this proposal is due to the translational nature of the work on compound optimization that may impact many diseases related to CCR2. The project is innovative because this is the first time the structure of CCR2 will be determined, and because of the focus on novel and mechanistically diverse inhibitors of this receptor that may lead to better in vivo efficacy.